Strains of Prochlorococcus cyanobacteria contribute nearly half of the photosynthesis in the open ocean. Different ecotypes have distinct morphologies. The tiny and spherical MED4 strain holds a 1.66-Mb genome and appears to prefer surface waters. In contrast, the larger (2.41-Mb genome) and ovoid MIT9313 ecotype is abundant in deeper (>50 m) subtropical and tropical waters.

By rapid freezing of hydrated cultured cells and cryoelectron microscopy, Ting et al. were able to document a substantially divergent cellular organization and structure in these bacterial strains. The MED4 strain possesses a thinner cell wall and a less extensive intracytoplasmic (photosynthetic) membrane system in comparison to MIT9313. The authors also find differences in key genes required for the biosynthesis of the cell wall peptidoglycan, where the greater similarity of these genes in MIT9313 and Synechococcus WH8102 correlates with their much thicker peptidoglycan layers. Overall, it seems that MIT9313 cells are better adapted for photosynthetic growth at low irradiance levels in deeper waters, although nutrient transport could also influence cell size and shape. — CA